Bisphenol-A (4,4′-dihydroxy-2,2-diphenylpropane or BPA) is produced by condensation of acetone with an excess of phenol in the presence of an acidic catalyst or a cation-exchange resin. The crude product, in addition to the desired bisphenol-A and unreacted phenol, contains unwanted by-products, such as bisphenol-A isomers, trisphenols and other higher molecular weight materials. The bisphenol-A is normally separated from the crude product by a single or a series of crystallization steps, leaving a mother liquor stream enriched in unwanted by-products, a portion of which stream is removed to purge unwanted by-products from the process. Alternately, the bisphenol-A may be separated from the crude product by a single or series of distillation steps, which also creates a stream enriched in unwanted by-products, a portion of which is removed. The removed stream may contain unreacted phenol and bisphenol-A as well as the unwanted by-products. Phenol is typically recovered from the removed stream by distillation, normally vacuum distillation, to create a residue stream concentrated in unwanted heavies which is purged from a BPA manufacturing process.
There is substantial prior art describing methods for recovering phenol and isopropenyl phenol from such residue streams to improve the economic performance of the overall BPA manufacturing process. One such method is described in U.S. Pat. No. 6,191,316 and involves addition of catalytic amounts of base at elevated temperature under vacuum to decompose BPA isomers, trisphenols and other by-products into phenol and isopropenyl phenol followed by addition of catalytic amounts of acid at elevated temperature under vacuum to recover phenol.
In addition, there is substantial prior art describing methods for recovering phenol and acetone from BPA and BPA residue streams. One such method involves hydrolysis of BPA and BPA residues purged from a BPA manufacturing process in the presence of water at supercritical or near-supercritical temperatures and pressures (see, “Phenol Recovery by BPA Tar Hydrolysis in Supercritical Water”, Adschiri T., Shibata R., Arai, K., Sekiyu Gakkasishi, Vol 40, No. 4, 1997, p. 291-297).
Moreover, hydrolysis of BPA and BPA residue has been shown to occur at subcritical temperatures and pressures in the presence of an aqueous solution of ammonia, alkali-metal and alkaline earth metal hydroxides and carbonates to produce phenol and acetone which can then be recovered (see U.S. Pat. No. 3,075,015). In this process, the concentrated heavies are reacted with sodium hydroxide solution or other basic solution to convert the p,p-BPA and other compounds back to phenol and acetone. The acetone is recovered in a distillation column and the phenol is recovered by neutralization followed by steam distillation. Phenol and acetone yields using hydrolysis are substantially improved compared to methods using catalytic decomposition in the absence of water.
However, the '015 patent is silent as to subsequent treatment or disposition of the remaining aqueous mixture or the unhydrolyzed heavies. Therefore, to achieve economic benefit from hydrolysis of the BPA isomers and impurities in the residue stream, a solution is required to efficiently recover and recycle acetone and phenol from hydrolysis of the concentrated heavies to the BPA manufacturing process, to efficiently dispose of the resulting aqueous mixture, and to efficiently separate the unhydrolyzed heavy organic compounds from the aqueous mixture. The present invention seeks to provide such a solution.